So, again, are these units replacing metal detectors as the primary screening method, or are they just going to be used if you get a secondary?

If it's to replace metal detectors, whoever is thinking they won't slow down screening is a total moron. They will at least quadruple the total amount of time it takes to get through a given line, compared to now. Walking through a detector takes a second. Walking to the machine, getting in proper placement, then turning on the juice, then waiting for the all-clear from the bozo in the room, hell, more like 10 times as long as a metal detector and that counts time spent in line after putting your stuff through the x-ray but waiting for the smurf to wave you through.